When producing polyurethane foams, at least one of the reactive components (polyisocyanate and compounds which contain isocyanate-reactive hydrogen atoms, in particular polyols) is sometimes mixed with a liquid or gaseous blowing agent and then mixed with the other component to make foam.
The blowing agent is employed in order to enhance the expansion of the foam which is obtained from the reaction of the water with the isocyanate. Blowing agents used are either low-boiling liquids such as low-molecular chlorofluorocarbons, methylene chloride, pentane, and the like, which evaporate due to the increase in temperature of the reactive mixture in the course of the exothermic polyaddition reaction, which chemically releases carbon dioxide due to the reaction between the water and the isocyanate.
For reasons of environmental compatibility and occupational hygiene and because of the comparatively high solubility of liquid carbon dioxide in the polyol component, liquid carbon dioxide has already been proposed as an auxiliary blowing agent on several occasions.
Polyurethane foams using CO.sub.2 as an auxiliary blowing agent and produced using impingement mixing, have shown a tendency towards voiding or "pin-holing" making this approach limited in commercial application.
Liquid carbon dioxide as a blowing agent has been suggested, for example, by Fiorentini, et al. (U.S. Pat. Nos. 5,578,655, 5,629,027 and 5,620,710) and Doyle, et al. (U.S. Pat. No. 5,120,770).
It is known to use equipment consisting essentially of a standard molded foam machine design with the addition of a high pressure CO.sub.2 addition unit. The CO.sub.2 is injected into the polyol stream just before it enters the mixhead. Even though the CO.sub.2 is maintained as a liquid, there is not enough time for the CO.sub.2 and polyol to become a homogenous mixture before it reaches the mixhead which often results in voiding or pin-holing.
It is also known to use a process comprising a CO.sub.2 cylinder and a high pressure day tank fitted with a gas nucleation device. In this method, the machine is fitted with a higher pressure (300 psi max) CO.sub.2 addition unit, a high pressure day tank, and a static mixer (in the recycle loop to the mixhead). In this method, the CO.sub.2 is pumped into the static mixer in the recycle line, CO.sub.2 flow is discontinued when the CO.sub.2 pressure solubility has reached the desired equilibrium point. Once this has been completed, the polyol tank pressure is further increased with N.sub.2 to ensure that the proper amount of CO.sub.2 remains in solution. Since CO.sub.2 gas is present in the polyol tank to maintain this equilibrium, all of the gas is not in the liquid state.
Another approach, differs from the previous approach by having the CO.sub.2 introduced into the isocyanate tank instead of the polyol tank.
Fiorentini, et al. discloses a continuous slabstock foam process. The CO.sub.2 is injected at high pressure into the polyol system as it is being pumped to the mixhead (with a mechanical agitator). The mixture passes through a static mixer, a collector block where the foam activators are added and finally into the mixing chamber where the polyol and isocyanate are mixed. Before being dispensed onto a conveyor, the foam mixture passes through a pressure let-down device, which is not required in the present invention, at the end of the mixhead, which reduces the tendency towards voiding.